I always strive to be clear so that everyone can follow the debate, and to strip down the arguments to their essentials so people don't forget to see the forest for the individual trees, so I don't think I can express my take on this more clearly than I already have...
However, I also strive to be polite, so I'll have one more go at it.
They were successful because they were in the right place(s), at the right time, with the right tools, and the right social organization, as is true for any group that becomes dominant.
The tools, except for the domestication of the horse, and perhaps their more than average liking for warfare and development of a warrior cult, and their extremely patriarchal society, they initially adopted from others in a rather wholesale fashion. There's nothing in the archaeology or anthropology to indicate that the parts of their "package" that relate to farming, animal husbandry or herding, mining, metallurgy etc. were developed on the steppe . The steppe peoples had none of those things, and adopted them from cultures which had been developing them for hundreds if not thousands of years. There's absolutely no question about that. If you think there is, it's because you haven't done enough reading on the subject. Even carts, even war carts, are earlier in either Old Europe or the Near East.
There's nothing wrong with building a new civilization based on the technology of others, and then improving on it to some extent, by the way, as they did when they perhaps invented the spoked wheel and attached it to a cart and a horse in 2000 BC in Sintashta, or in those later periods became metal smiths of some repute. It shows realism and practicality and flexibility. That's what the Japanese have been doing since the 1800s. To some degree, it's what the Romans did by building on the accomplishments of the Greeks, and the Greeks by building on the accomplishments of the Near East, and so on. More power to them.
The right tools also refers to the fact that in many of the places they arrived, the "package" they had put together was new to the area and offered a better way to utilize the ecosystem. That wasn't true everywhere, however, which may have been a factor as to whether their alleles are a small or a larger proportion of the modern total, although there were other factors, like population density, no doubt.
The right time refers to the fact that Central Europe, for example, had experienced repeated population crashes because of climate change, deforestation and other environmental degradation because of slash and burn farming methods, and the unsuitability of this early form of farming to the climate and terrain of central and northern Europe. These areas were only able to support larger populations thousands of years later in the Middle Ages when different types of plows were invented which could turn the heavy earth, and they learned about crop rotation and other things. They also needed slightly different cereal crop strains. This is all well known. Please use the search engine to find the citations. It will be worth it; it's all very interesting. I just can't spend any more time on this by also hunting down the citations; there's this little thing called my life that I sometimes remember.
We also know that they did indeed carry the plague. (Again, use the search engine.) It's perhaps somewhat analogous, although not totally, to the situation in the Americas when the Europeans brought the measles, chicken pox, small pox, etc. to the New World. It made the Europeans sick, killed a good number, but it decimated the native Americans. In some places 90% of them died.
As to why certain areas of Europe, predominantly central, eastern and northern Europe, are so heavy in the "Indo-European" clades of R1b and R1a, the preceding should make the answer self-explanatory. If you come in to a sparsely settled area or one that has seen a population crash, and you happen to also harbor plague, and of the surviving population, you either kill a lot of the men, or otherwise make them disadvantaged for procreation (enslaving them or making them serfs?), and so the vector for the genes of the earlier inhabitants is far more often the women, then would you expect anything else? I wouldn't.
In the Americas it was even more extreme in certain areas, even in relatively advanced areas like those of the Aztecs and Mayas, because the disparity in technology was still greater. In addition to disease, and horses, and steel swords, they had guns and cannon, for goodness sakes. Hardly an "equal" fight. The native civilizations had agriculture, and some metallurgy, but they didn't have domestic animals for riding, or iron and steel yet, and certainly not guns. That's why so many of the men in Latin America carry European yDna bub native mtDna.
Have you read Guns, Germs and Steel by Jared Diamond? I think you'd find it very interesting. (I don't agree with all of his proposals, but it's important to read him, I think.)
https://en.wikipedia.org/wiki/Guns,_Germs,_and_Steel
I do think we badly need some research into the effects of various mtDna on health, fitness, and procreative ability as well. We know how important mtDna is for all of these things. There's already a bit of research that mtDna "H" is better for protection against sepsis and women who have it bear more children, likewise some indication that carriers of some R1 clades may be slightly more likely to sire sons, and even a slight propensity could have a maximum impact over time.
So, based on the papers and books we have so far, that's my take on it. I'm always more than ready to change my position as new evidence comes in.
I don't think there's any purpose served by debating this with you any more. You can accept it or not, as you choose. I would advise that you look for the papers, however.
